The present invention relates to a time division multiplexing amplifier which is used to amplify differential signals, and more particularly to a differential amplifier providing a high common-mode rejection ratio.
In the case where a transducer is provided with its output stage having a balanced-output type circuit arrangement like a bridge circuit or the other devices like a thermo-electric device, from which a very small differential output signal is derived, the differential output signal frequently involves a relatively large amount of common-mode noise voltage with respect to ground and, therefore, it is necessary to eliminate the adverse effect of the common-mode voltage in order to accurately amplify the differential signal voltages. For example, in order to amplify a 10 mV differential signal voltage involving a 10 V common-mode voltage with accuracy of 0.1%, an overall common-mode rejection ratio of the amplifier is required to be as high as 10 V/10 .mu.V = 1,000,000 or 120 db since 0.1% of 10 mV is 10 .mu.V.
If there are a plurality of signal sources, a single differential amplifier has to amplify a plurality of signals supplied from these sources through a switching device, but a different signal source has a different common-mode voltage. Therefore, if the common-mode rejection ratio for a particular common-mode voltage is improved, the improvement is effective only for the particular signal source and never for any of the remaining sources. It is also necessary to consider the adverse effect of stray capacitance and leakage resistance of the circuit on the performance of the amplifier, since signals to be processed are very weak.
The conventional method of amplifying differential signal voltages containing common-mode voltages may be generally classified into the following two types:
One method is to amplify the differential signal voltages by means of an isolated amplifier: This method uses elements which handle signals in an electrically isolated condition, such as pulse transformers and semiconductor photo-couplers, so that the input stage of the amplifier or the whole amplifier is isolated from the common-mode voltage. In this method, high common-mode voltage inputs are allowable because some insulating parts are capable of withstanding high voltages. However, the use of the pulse transformer isolation may not offer sufficiently high differential input impedance, while the use of the semiconductor photo-coupler isolation may not offer sufficient linearity in signal transmission. Therefore, accurate amplification may not be achieved in either case. In addition, this method will cause the circuit to be complicated and costly.
Another method is to amplify the differential signal voltage by means of a direct-coupled non-isolated differential amplifier: This method uses a feedback amplifier which has a balanced input stage adapted to present high input impedance against the common-mode voltage and also which provides accurately a predetermined amplification factor for the differential signal voltages. In general, this type of differential amplifier is called an instrumentation amplifier or data amplifier; for example, a dynamic bridge amplifier composed of three differential input operational amplifiers and four or more precision operational network resistors. In this type of differential amplifier, the common-mode rejection ratio, which represents the degree of elimination of the adverse effect of a common-mode voltage, depends on the common-mode rejection ratio of the differential input operational amplifier to be used and also on the accuracy of operational resistors to be used. Various attempts have been made in the past to adjust both amplifiers and resistors so as to obtain a maximum common-mode rejection ratio, but it has been still difficult to obtain a stable and high common-mode rejection ratio.